Conventional processes for the assembly rework of surface mount technology (SMT) boards typically comprise removal of the components and cleaning of the sites. To fix the replacement components to the boards, one or more flat mini-metal stencils are applied to the boards to define the solder pattern to be applied. Solder paste is then applied to the solder pads on boards and is forced through the apertures in the stencil(s) using a squeegee. The one or more stencils are then removed. The semiconductor device is then positioned on the board and fixed thereto by reflow soldering of the solder paste.
The flat mini-metal stencil is custom-sized to fit into the reworked space within the confines of surrounding components. The mini-metal stencil may be formed with sides and other features that allow for manual mechanical fixturing for alignment to the printed circuit board (PCB) pattern. However, these mini-metal stencils lack the mechanical support of the rigid stencil frame, as well as the automated functions of alignment, contact pressure, squeegeeing and stencil lift off that make the use of metal stencils successful in the primary SMT printing function.
In such conventional processes, the flat mini-metal stencil is positioned by manual alignment and is taped along the stencil's edges to the PCB. Alternatively, the formed mini-metal stencil may be aligned with a mechanical arm, which is required to be set-up for each component location. Both of these methods can result in insufficient mechanical retention between the stencil and PCB surface. Other problems with such conventional processes are that the stencil position may shift during squeegee passes, and the stencil may need to be removed and the board require cleaning and re-stencilling. Furthermore, alignment and positioning are typically difficult in such processes. Also, mini-metal stencils must be cleaned thoroughly between each use.
Further difficulties with the mini-metal stencil method include the possibility of not having enough solder paste deposited to form the contact pad or of having solder paste bleed under the stencil.
As the stencil is made of metal, it is not easy to tailor the stencil to fit the reworked site, which is typically a confined spaced between other semiconductor devices. As a result, the use of the mini-metal stencils requires a high level of skill to ensure that the correct amount of solder paste is deposited with only a single pass of the squeegee.
An alternative conventional method is to use a mini-metal stencil to apply the paste to the semiconductor device contacts and the thermal pads on the semiconductor device, rather than to apply the stencil to the land of the printed circuit board as described above. Similar difficulties arise in this alternative method, as are mentioned above.
A number of the above-mentioned problems with prior art systems are addressed in U.S. Pat. No. 6,253,675 issued to Mayer (the '675 patent), which describes a solder paste stencil printing apparatus and method. The stencil is a disposable, adhesive-backed, flexible, polymer membrane rather than being formed of metal. The stencil adheres to the PCB whilst the solder paste is applied thereto and squeegeed across the stencil surface. The stencil is then removed and the semiconductor device is located manually on the solder paste. A reflow process is applied to melt the solder and fix the semiconductor device to the PCB.
Although the device and methods described in the '675 patent address some disadvantages associated with conventional techniques using metal stencils, such as fixing the stencil to the PCB during the squeegee passes to prevent relative movement and inhibiting the solder paste from seeping under the stencil, the systems and devices described in this patent may still suffer from a number of disadvantages. For example, the alignment and positioning problems described above in connection with metal stencil techniques still exist, as well as the possibility of damage during stencil removal. Furthermore, a high level of skill is still required to ensure that the correct amount of solder paste is deposited with only a single pass of the squeegee.
U.S. Patent Application Publication No. 2001/0000905 A1 (the '905 publication) describes a system for ball grid arrays in which the conventional stencil is replaced with a preformed alignment device having a pattern of holes therethrough that may be aligned with the pattern or footprint of the ball grid array. The holes are filled with either solder paste, a curable conductive adhesive, or solid solder.
In use, the preform described in the '905 publication is attached to a PCB and the semiconductor is then placed above the preform so that the preform is interposed between the semiconductor being placed on the PCB and the PCB, with the material in the filled holes being in contact with the solder balls on the semiconductor device and the contact pads on the PCB. Heat is applied to reflow the solder or cure the adhesive in the holes to join the semiconductor device to the PCB. The preform may be left in place on the PCB thereby eliminating the possibility of damage that may occur in other conventional systems where the stencil is removed. Alternatively, if it is desired to remove the preform, a suitable material that can be dissolved away after the reflow process is used to form the preform. However, the alignment problems described above in connection with metal stencil techniques still remain.
In view of the foregoing problems with conventional processes and devices, a need exists for a quick and an easily applied method for replacing individual leadless semiconductor devices, such as QFN devices.